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Old 2nd May 2007, 12:24 PM   #1
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Default Wiki on bracing

I think it’d be useful to gather and summarize the wisdom into a wiki “bracing: theory & practise”.

I have a mix of knowledge and questions, gleaned solely from searching here, most from dave, some from GM and others.

A 1st draft, probably with a number of misconceptions, hopefully not too many, and IMO lacking some bits to be in a wiki just yet(?):


The goal of bracing is to push panel resonances higher. Ideally the natural frequency of the panel is pushed above and outside the passband of the box, eg above a dedicated mid box’s passband.
Where this is not possible, next best is simply to reduce its audibility, as the higher the frequency, the greater the amount of energy needed to excite it.
Also damping materials attenuate higher frequencies better.

The Hz of a panel resonance is inversely proportional to the square (?) of the panel size, eg if a panel size is doubled, the frequency of a resonance increases by a factor of 4(?).
If the frequency of a resonance increases by a factor of 4, the energy needed to excite it falls to 1/ 4 (or 1/ 16??).

As mass stores energy, the releases it with a time delay, a well braced single layer box is better than a non-braced two layer box.

Theory
From "Vibration Analysis for Electronic Equipment", by David S. Steinberg (p 267):

f = (pi/ 2) * SQRT{D/ p[(1/a^2) + (1/ b^2)]}

where

f = natural frequency
D = plate stiffness factor = [E(h^3)] / [12(1-u^2)]
E = modulus of elasticity in lb/ in^2
Note 3/4" no-void ply has an MOE of 1.8 mill psi,
while typical MDF is 0.53 mill psi.
h = plate thickness, inches
u = Poisson ratio

p = mass per unit area = v * h / g
v = material density in lb/in
h = plate thickness, inches
g = acceleration of gravity, 386 inch/sec^2

a = length of the plate, in inches
b = width of the plate, in inches.

The frequency is proportional to the thickness to the 3/2 power, the SQRT(MOE) and to 1/SQRT(mass).


Design
In setting the box dimensions, allow 5-10% for the volume of the braces.


Spacing
· The largest panels resonate at the lowest frequencies, so they are the most important to divide up
· Linkwitz’ rule of thumb from years ago: no unbraced distances should be greater than 4" ie 100 mm.
· GM for void free plywood suggests the points where bracing around the driver is required:
- for 5/8" ply: 0.75 ft^2 CSA (cross sectional area), ie equivalent to 10” ^2, ie 25 mm^2.
- For 3/4" ply: for 1.0 ft^2 ie 30 mm^2.



Shape
· the strongest (bracing) structure is a triangle . .
·
·

Placement
· Bracing the short dimension creates a larger number of panels that are closer to square, with same or similar resonances. It’s better to run them along the long dimension of a panel.
· the middle is the worst place, as energy at divisor frequencies can feed panel resonances - the panel can actually be worse after bracing. To spread resonances, bracing should never be at equidistant intervals, eg instead 1 ft intervals - 11", 13" & 12" would be better.
· So that the 1st resonance is killed instead of enhanced - place them offset and/ or oblique ~ by at least X? degrees
· brace so that you move the dimensions away from each other
· endeavour to brace the driver magnets to the box.

Materials
· plywood or hardwood, because they are stiff and not heavy (mass absorbing energy, then releasing it later).
· A good size is 3/4" x 1 1/2" ie 18 * 36 mm.
· mount edge-on, ie deeper side attached to the box.
· metal braces or steel rods are stiffer so even better

Tips for attaching braces
· Butt joints with glue & screws is fine.
· Ideally each bracing sheet is connected to other bracing sheets(?)

· If the bracing also ties the box walls together, that is better
· Best - run rods all the way thru, and add compression to the panels (ie try to bend the walls into a dish shape).

Shape
http://www.diyaudio.com/forums/showt...ht=#post832740

· holes in the braces - are to . . . ?
· Approx 30 - 60% is a good proportion of holes

· Shelf braces ( . . . . . ) are not effective because . .
· Instead, braces should divide the panels into long thin unequal, preferably trapezoidal sub-panels

Boxes for full-range or midrange-&-above :

. . . .

And for subs (XO < 100 Hz):
At 100 Hz, as long as the biggest box dimension is < 1.7 m (5.6’), there is no benefit to bracing, as potential panel resonances will be above the panel size.
See also this short thread on push-push:
Push-push phase and this wiki
http://www.diyaudio.com/wiki/index.p...river+mounting


Please feel free to correct me, let me know if you think it’s ready to be wiki-ed

Cheers
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Old 2nd May 2007, 04:04 PM   #2
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Default Re: Wiki on bracing

Quote:
Originally posted by rick57
The Hz of a panel resonance is inversely proportional to the square (?) of the panel size, eg if a panel size is doubled, the frequency of a resonance increases by a factor of 4(?).
Others may correct me but if the panel size is doubled, wouldn't that lower the resonant frequency?
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Old 2nd May 2007, 04:11 PM   #3
Zarathu is offline Zarathu  United States
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Default hardwood dowels?????

So you are saying that cross-through hardwood dowels placed in multple spots is the most effective?

What about screw adjustable cable or chain which puts an inward concave pressure on the box sides? This is done in houses to bolster against earth quakes.

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Old 2nd May 2007, 10:58 PM   #4
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Where is the Wiki located...or are you going to post it at Wikipedia?
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Old 2nd May 2007, 11:08 PM   #5
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Guys,

Please note, “gleaned solely from searching here” ie I have 0 claim to expertise.

I started simply to gather info for my own understanding and use; then thought - why not share it. I know nothing beyond what I posted.

. . also “if you think it’s ready to be wiki-ed”

Dave, as a moderator – wiki it now?

Cheers
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Old 2nd May 2007, 11:31 PM   #6
Zarathu is offline Zarathu  United States
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I noted your discussion on the parts Express forum and a number of the EE types they had comments.

You could read the comments.

Here: http://www.pesupport.com/cgi-bin/config.pl?read=350291

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Old 2nd May 2007, 11:40 PM   #7
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Hi Rick,

Just wanted to say thanks for putting the time and effort into doing this. Perhaps less is more in this case and people could add to it later on.

Cheers,
Martin.
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Old 2nd May 2007, 11:45 PM   #8
Ron E is offline Ron E  United States
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Default Re: Wiki on bracing

While I admire the enthusiasm, this is definitely not ready to be wiki'd. One could go through and pick this all apart, but I won't.

I think it is good to have a plate resonance equation. Make sure to work though an example and make sure everything works correctly. You might want to put a membrane resonance equation there and discuss the frequency region where it is applicable.

There is a limit to how effective even herculean efforts can be. Take a look at B&W 801 review in stereophile and note what the effects of matrix bracing are. The box still resonates at a surprisingly low frequency. The amplitude is satisfactorily low, though.
http://stereophile.com/floorloudspea...06/index7.html
When looking at (comparing) these plots you have to be very careful to understand where the accelerometer was placed. I wonder how JA chooses a location?

A (box) speaker is more than just 6 panels, there is complicated 3D motion going on (twisting, etc) that requires FEA to analyse.

Point bracing is nowhere near as effective as shelves or ribs.

IMO complicated bracing structures are probably more likely to rattle.

There are too many unsubstantiated general statements in your text. You would do well to track down supporting literature, etc.

Cheers.


Quote:
Originally posted by rick57
I think it’d be useful to gather and summarize the wisdom into a wiki “bracing: theory & practise”.

Please feel free to correct me, let me know if you think it’s ready to be wiki-ed

Cheers
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Old 2nd May 2007, 11:46 PM   #9
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Default Re: Wiki on bracing

Quote:
Originally posted by rick57
Where this is not possible, next best is simply to reduce its audibility, as the higher the frequency, the greater the amount of energy needed to excite it.
No time to go thru the whole yet, but this should be "there is less energy available to excite the resonances"

dave
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Old 3rd May 2007, 03:03 AM   #10
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Originally posted by rick57
Where this is not possible, next best is simply to reduce its audibility, as the higher the frequency, the greater the amount of energy needed to excite it.

posted by dave
No time to go thru the whole yet, but this should be "there is less energy available to excite the resonances"

dave
Is it mostly your point about energy available, but also that a greater amount of energy is needed to excite it?
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